The invention relates to the field of mobile telephone systems. A subscriber of a mobile telephone system, that is, a subscriber station, e.g. a mobile telephone or a mobile station may be connected to a mobile network or system, whereby it is connected to the mobile network via system channels maintained by the base stations of the mobile network, the system channels consisting of control channels and traffic channels.
In addition to the system channels located in mobile telephone systems, so-called direct mode channels may be used in association with the mobile system, i.e. communication takes place in a direct mode. Subscriber stations communicating in the direct mode do not communicate directly with the mobile network or the base stations related to it. Direct mode channels are frequencies on which mobile telephones or other means of communication can communicate without the system directly with each other, or via repeater stations, either with the base stations of the system or with other mobile stations.
Direct mode channels are typically used in situations in which e.g., a group of hand-held mobile phones which communicate with each other are so remote from the base stations that the use of system channels is not possible.
Another important use of direct mode channels is increasing the capacity when the traffic in the system rapidly increases, e.g. due to an incident occuring in some part of the service area of the system, e.g. in some spot-like part.
A direct mode channel is a channel that is typically not used by the system. It may be e.g., a channel whose channel spacing is the same as that in the mobile communications system, e.g. 12.5 kHz or 25 kHz. On direct mode channels, a mode of simplex operation is used. From the group of the mobile stations operating on the direct mode channel, the transmitting party has tuned his or her transmitter onto the channel and is transmitting speech or data information. The other mobile stations that have been activated onto the direct mode channel have tuned their receivers onto the same frequency so that they will directly hear the transmission.
On a direct mode channel, communication may take place by means of analog modulation or digital modulation. A mobile station transmitting on the channel may also use signalling information, such as information on access rights and priorities, or the group communicating on the channel, or data communication. On a direct mode channel, encryption may be carried out, or plain speech can be transmitted.
Subscriber stations operating in the direct mode communicate with other subscriber stations on a direct mode channel without having a direct connection to the base stations of the mobile network. Subscriber stations operating in the direct mode, however, may communicate with the mobile network via repeater stations. A repeater station is a unit in which there are two transceivers connected to each other. A repeater station typically comprises two transceivers. The repeater station forwards information messages transmitted on the direct mode channel to the desired network elements, such as the base stations of the radio network, which will forward the information messages to the exchanges of the radio network. Information messages forwarded by the repeater station may be e.g., speech, or data or signalling messages. When the repeater station is being used, e.g. a semiduplex mode of operation may be used on the direct mode channel.
A problem related to a prior art solution is how to make the mobile station operate upon request in accordance with the dual watch mode, that is, to communicate "simultaneously" on the direct mode channel and the system channels of the mobile communications system. A mobile station operating in the dual watch mode on the direct mode channel may receive transmissions of the system channels of the mobile communications system between time-slots received on the direct mode channel. A dual watch function of this kind has not been implemented before between a mobile station operating on the direct mode channel, and a mobile communications system. This kind of a situation is attended by the drawback that the frame synchronization of the direct mode channel will be changed with respect to that of the mobile communications system, in which case it is difficult or even impossible for the mobile station to make use of the dual watch mode.
Another problem related to prior art solutions is the fact that the mobile communications system does not know whether the mobile station is in the dual watch mode or not, in which case it is difficult for the system to co-operate with the mobile station possibly operating in the dual watch mode.
Yet another problem related to prior art solutions is the fact that the mobile communications system does not know the identifier of the direct mode channel on which the mobile station in the dual watch mode is operating.